Chronic muscle pain syndromes (e.g. low back pain, myofascial pain, and pain induced by repeated exertion or exposure to mechanical vibrations), are common, debilitating, and very costly to society. Chronic muscle pain remains extremely difficult to treat, in large part due to the lack of understanding of underlying mechanisms. Much current muscle pain research focuses on the effects of strain on muscle and tendon tissue, but does not address pathophysiological mechanisms specifically responsible for chronic muscle pain. Importantly, virtually nothing is known about the cellular mechanisms underlying the critical transition from acute to chronic muscle pain. We propose an approach that will directly target the pathophysiological mechanisms of muscle pain, focusing on the transition from acute to chronic pain states. This investigation will integrate behavioral, electrophysiological and cell biological methods to link observations on cellular mechanisms to pain-related behavior in the animal. In Specific Aim 1, we will characterize the acute pain state, identifying the pronociceptive effects of two mediators and second messengers produced by abnormal activity in muscle that affect primary afferent nerve fiber function. In Specific Aim 2, we will investigate the cellular mechanisms engaged by the acute pain state that cause it to evolve into a chronic condition of prolonged hyperalgesia. We have already performed critical preliminary studies that have revealed a profound, long- lasting muscle hyperalgesia produced by inflammation and data that strongly suggest our approach can reveal fundamental mechanisms of muscle pain syndromes. In this application, we propose to study cellular mechanisms of the transition from acute to chronic muscle pain in animal models of muscle pain syndromes. These studies have the potential to identify novel targets for the development of new pharmacological therapies for the treatment of this debilitating condition.